1. Field of the Invention
The present invention relates to a flavor-retaining plastic multi-layer container. More specifically, the invention relates to a flavor-retaining plastic multi-layer container having an intermediate layer of a cyclic olefin copolymer and inner and outer layers of an olefin resin or an ethylene/vinyl alcohol copolymer, preventing degradation in the adhesiveness among the layers when the container is filled with a content having fragrant components.
2. Prior Art
Plastic multi-layer containers have been extensively used for containing a variety of contents such as toothpaste, cosmetics, toiletries, chemicals, pastes and adhesives, highly viscous seasonings, foods and the like. In particular, the squeezable containers must have flexibility for squeezing out the content, and have been obtained by using ethylene polymers such as low-density polyethylene, ethylene/vinyl acetate copolymer and the like.
The multi-layer contains are obtained by using various plastic materials in combination in order to satisfy the requirements of moldability, manufacturing cost and properties for preserving the contents. The inner and outer layers of the containers, in many cases, are composed of a linear olefin resin such as polyethylene or polypropylene from the standpoint of moldability, sanitation and cost.
In order to prevent the permeation of oxygen through the container wall, it has long been known to use an oxygen-barrier resin as represented by, for example, a multi-layer extruded tube container comprising inner and outer layers of a low-density polyethylene and an intermediate layer of a saponified product of an ethylene/vinyl acetate copolymer (ethylene/vinyl alcohol copolymer) (e.g., Japanese Patent Publication No. 33223/1982).
It has also been known to produce a multi-layer container by utilizing the fact that a cyclic olefin copolymer exhibits excellent resistance against the permeation of moisture. For example, Japanese Laid-Open Patent Publication No. 276253/1992 discloses a container for medicines or foods, comprising many layers including two layers of different materials, at least one of the layer being composed of a thermoplastic norbornene polymer. There has further been taught to use a polypropylene modified by chlorination or with maleic anhydride in order to adhere the layers together.
Japanese Laid-Open Patent Publication No. 52340/1995 discloses a multi-layer plastic container of a structure in which at least two or more kinds of resin layers are laminated one upon the other, the outer layer being composed of an amorphous resin obtained by the copolymerization of a cyclic olefin with an ethylene, and the inner layer being composed of a polyolefin resin. The layers are adhered together by using an ethylene/.alpha.-olefin copolymer modified with an acid.
Japanese Laid-Open Patent Publication No. 266517/1995 discloses a moisture-proof multi-layer extruded structure constituted by at least three layers of resins and having excellent shock resistance, in which a polyolefin resin layer containing 5 to 60 mol % of a cyclic olefin component is interposed between the polyolefin resin layers. In this prior art, there has been disclosed to use the olefin resin modified with acid in order to adhere the layers, too.
The present inventors have previously discovered the fact that the flavor-retaining property of the container can be improved without adversely affecting the resistance against the contents or the sealing property when a cyclic olefin copolymer is used as an intermediate layer between the layers which are usually composed of a thermoplastic resin such as olefin resin (Japanese Laid-Open Patent Publication No. 11416/1997).
However, it was found that when a multi-layer container having an intermediate layer of a cyclic olefin copolymer and inner and outer layers of an olefin resin or an ethylene/vinyl alcohol copolymer, is filled with a content containing fragrant components, the adhesion among the layers is degraded to a conspicuous degree with the lapse of time. That is, the cyclic olefin copolymer and the olefin resin (non-cyclic olefin resin) poorly adhere together, the cyclic olefin copolymer and the ethylene/vinyl alcohol copolymer poorly adhere together, and, hence, an adhesive resin is interposed between the two resin layers as taught in the above-mentioned prior arts. Here, however, the adhesive resin that has heretofore been used exhibits a high adhesive strength before the container is filled with a fragrant content, but loses adhesive strength conspicuously with the lapse of time after the container is filled, giving rise to the occurrence of delamination.
It would appear quite queer that the laminate is delaminated despite the amount of the fragrant components is very small in the content. This is attributed to that the cyclic olefin copolymer existing as an intermediate layer works as a barrier against the fragrant components, and the fragrant components that have permeated through the inner layer of olefin resin or ethylene/vinyl alcohol copolymer act upon the interface between the cyclic olefin copolymer and the adhesive resin, to deteriorate the adhesion.
In the squeezable containers such as extrusion containers, the plastic material constituting the container wall must have a flexural rigidity which is as small as possible and a softness, so that the content can be smoothly extruded preventing the suck-back of the air (suction of the air by the container). For this purpose, a low-density polyethylene (LDPE) has been widely used as a material for constituting the walls of the squeezable containers.
On the other hand, the barrier property of the cyclic olefin copolymer against the fragrant components and moisture varies depending upon the glass transition point (Tg) of the cyclic olefin copolymer; i.e., the cyclic olefin copolymer having a higher glass transition point exhibits better barrier property.
It was found that a problem arises in the molding when a multi-layer container is molded by coextruding a cyclic olefin copolymer having a high glass transition point and a low-density polyethylene. That is, the cyclic olefin copolymer having a high glass transition point must be extruded at a high temperature. Accompanying a rise in the extrusion temperature, however, the low-density polyethylene develops draw-down, causing the thickness of the extruded product to be deviated, thickness of the walls to become nonuniform, and appearance of the containers to be deteriorated.